Assuming you have auto map update on (most do) its easy on the chart.
Zoom in real close to it, use the pencil to create a mark, at the same time start a stop watch (in game or a real one).
Leave it going for 1 minute, 2 is far better.
Then using the ruler tool measure the distance between those points.
Assuming you're using imperial you'll get a distance travelled in nautical miles per time interval.
If your target for example travelled 0.5nm in one minute then just 0.5 x 60 minutes to give its speed, in this case 30kts
If you used 2 minute (better due to course resolution of measuring ruler only doing 10ths of a mile) just do the same but divde by 2. Example its travelled 0.4nm in 2 minutes
(0.4 x 60) / 2 = 12kts (or just 0.4*30)
This works for preplanned attacks on convoys etc when you have time to do it.
Once detected though all bets are off as headings, speed and everything changes constantly so its guesswork and luck from then on.
You do NOT need to know AOB or anything else to use this method of calculating speed.
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